THE PEARLY NAUTILUS: SOME HOMOLOGIES 

 BETWEEN FOSSIL AND LIVING FORMS. 



Bv A. K. HORWOOD. 



It is well-known that the Pearly Nautilus* is tlie 

 single living survivor (generically) of a once 

 numerous and diverse group of Cephalopods which 

 commenced with a prototype, possibly like Tentac- 

 iilltes, having a 

 straight shell. More- 

 over, it .las been 

 established ihat the 

 race evolved from a 

 straight shell such 

 as Urtlioceras (sc(.' 

 Figure 399), a type 

 having a slightly- 

 curved shell like 

 CyrfocL'n7s (see Fig- 

 ure 400) ; and that 

 in course of time 

 this was again suc- 

 ceeded by a loosely- 

 coiled form which 

 had a spiral shell in 

 which the w ..oris 

 were not in contact, 

 like Gyrocems (see 

 Figure 401). Finally, 

 there was a type in 

 which the shell was 

 closely-coiled, and in which later whorls over- 

 lapped the earlier ones, and of such a type was 

 the early Nautilus (see Figure 402), whose descend- 

 ants inhabit the Indian and Pacific Oceans to- 

 day in pelagic depths. Such, brieily, is the course 

 of phylogeny or race-history amongst the tetra- 

 branchiate types of Cephalopods, and it is due to 

 the researches of many palaeontologists that we 

 now know that the life-history of a Cephalopod is 

 a repetition of that of the earlier race-groups. 



No one contributed more towards the establish- 

 ment of this great principle than the late Professor 

 Alpheus Hyatt. He showed, amongst other things, 

 that in the early stages of Nautilus it went through 

 a Cyrtoceras-Vike stage (see Figure 400), when the 

 shell was in shape like a ram's horn, uncoiled ; 

 and this stage was only the successor to a previous 

 straight stage in which the protoconch was 

 straight, and in line with the first chamber. But 

 unfortunately this protoconch is only revealed in 

 living as well as fossil forms by the scar left by 

 its junction with the first or initial chamber : for 

 the protoconch itself, possibly membranous, has 

 disappeared. 



FiGU 



A. Liassic Nautilus striatus 



J. de C. Sow., showing "black 



layer " and annular lobe. 



Man\- other facts have been elicited by a 

 study of the fossil Cephalopoda which all tend to 

 corroborate this broad principle of the recapitu- 

 lation of race characters h\- the individual, 



svhicli support in a 

 very convincing and 

 empirical manner 

 the still wider and 

 more general prin- 

 riple of progressive 

 evolution. That 

 there has been 

 retrogression, too, 

 this very group, the 

 Cephalopoda, bears 

 out in a remarkable 

 degree, not only in 

 the senile stages 

 acquired by the 

 individuals in both 

 two- and four-gilled 

 groups, but also in 

 the race - history, 

 where the former 

 became exceedingly- 

 abnormal, retro- 

 gressive, and finally 

 extinct, in cretaceous times. 



With this in view it is not remarkable 

 that both fossil and living forms are found to 

 exhibit in an extraordinary manner ver}^ perfect 

 homologies between the organs adapted for 

 any particular function in past and present 

 times, and in the effects of their action. And 

 no group of Cephalopods exhibits this charac- 

 teristic more markedly than the Nautiloids; for 

 in a considerable number of cases the means by 

 which the animal was attached to the shell 

 bv shell-muscles in the last or living-chamber, 

 and the marks left upon the shell-wall by 

 the muscular impressions, was almost exactlj' 

 identical in past times (as in the Jurassic period) 

 as to-day. 



And when we consider the extremely specialized 

 nature of the habitat of the Pearly Nautilus — more 

 or less deep-sea conditions, where little or no variation 

 in depth, light, warmth, currents, or composition of 

 the water could be effected — it is intelligible that the 

 mode of attachment and similar characteristics of 

 this interesting species should remain practicall}- the 

 saine throughout a vast lapse of time. For it is 



B. Young Shell of living Xauti- 



lus pompilius L. showing 



"black layer" (lateral view). 



There are four living species. 

 365 



